1. Field of the Disclosure
The disclosure relates generally to radiation detection and, more particularly, to three-dimensional, ionization-based radiation detection.
2. Brief Description of Related Technology
In view of the variety of ways in which radiation is generated and encountered, radiation detectors have been used to determine a number of different characteristics of the radiation, including radiation type, energy, source (e.g., the isotopes emitting the radiation), source intensity, and source location. The two main types of radiation—neutral particles such as photons (x-rays and gamma rays) and neutrons, and charged particles such as fast moving electrons and protons—have generally been detected with three types of radiation detectors, namely gas, scintillation and semiconductor detectors.
When gamma rays interact with a detector medium, charge carriers (e.g., electrons) are generated via electron ionization. The initial kinetic energy of the electrons equals the energy loss of the gamma ray. Furthermore, the number of created charge carriers is proportional to the energy deposition of each interaction. Both negative and positive charge carriers, such as electrons and holes in a semiconductor device, then move toward, and are eventually collected by, an anode (a positively biased electrode) and a cathode (a negatively biased electrode), respectively. The induced signals on the electrodes are proportional to the number of charge carriers. As a result, the amount of energy deposition has generally been determined by measuring the amplitude of the induced signal on an electrode.
Semiconductor detectors are favorable in gamma ray detection for their high atomic number, high density and low ionization energy for generating each free-moving charge carrier. Unfortunately, each currently available semiconductor medium presents limitations. Silicon detectors, for instance, have fairly low atomic number and the typical thickness is only a few millimeters. With the resulting low detection efficiency for gamma rays, silicon detectors are normally used to detect x-rays and charged particles. Other options include high-purity germanium detectors, which present a modest atomic number and good density, and can be produced in large sensitive volumes. While achieving excellent energy resolution and high detection efficiency for gamma rays, germanium detectors unfortunately require operation at liquid nitrogen temperatures to avoid spurious signals arising from a small band-gap energy.
Wide band-gap semiconductor materials, especially CdZnTe (CZT), have potential for both good energy resolution and compatibility with room-temperature operation. However, a number of challenges are presented by these room-temperature semiconductor detectors. Holes in CdZnTe move very slowly and are easily trapped, and thus contribute little, if at all, to the induced signal. As a result, the induced signal is mainly contributed by the movement of electrons, which, in turn, makes the signal amplitude dependent on the drift length of the electrons. Even for the same energy deposition, the induced signal then has a different amplitude depending on where the electrons are created. Various methods have been proposed and evaluated to overcome this problem, such as pulse shape discrimination, pulse compensation, and single-polarity charge sensing techniques. Unfortunately, the electrons can also be trapped during their drift to the anode, causing a deficit in the induced signal amplitude. Techniques using an optimized relative gain between two anodes and depth sensing have been proposed to address this electron trapping effect. Unfortunately, the energy resolution has remained far worse than the theoretical limit due to material non-uniformity.
Single-polarity charge sensing techniques, such as coplanar or pixelated anodes, have been utilized to minimize the hole-trapping problem and improve the energy resolution for larger volume detectors. Unfortunately, these techniques were still limited by problems arising from material non-uniformity and spatially varying electron trapping, thereby limiting the energy resolution of, for instance, co-planar grid detectors.
More recently, the foregoing challenges were addressed in the development of three-dimensional CZT spectrometers by He et al. and Li et al. See, for example, Z. He, et al. “3-D position sensitive CdZnTe gamma-ray spectrometers,” Nucl. Instrum. Meth. A, vol. 422, pp. 173-178 (1999) and W. Li, et al., “A data acquisition and processing system for 3-D position sensitive CZT gamma-ray spectrometers,” IEEE Trans. Nucl. Sci., vol. 46, pp. 1989-1994 (1999). By determining the three-dimensional (3-D) position information for a single-pixel event, the material non-uniformity and varying electron trapping effects were addressed. Unfortunately, these devices were incapable of correctly determining the information for multiple-pixel events.